Pavlovian Conditioning in Human Brains

Prof. John O'Doherty, California Institute of Technology, USA

John - What we're interested in is how can we learn to predict when good things are going to happen to us. It's very advantageous to be able to predict when either good or bad things are going to happen to us, because then we can prepare ourselves in advance and do the things that we need to do to get as much reward as we can. What we were interested in was the parts of the brain are involved in this learning process. We took a bunch of human volunteers and we put them in an fMRI scanner. What this scanner does is pick up changes in blood oxygenation, which are indirectly connected to neural activity. Therefore, we can actually see the bits of the brain that are being engaged during different types of learning process. What we did was presented the subjects with different flavours of juice, such as blackcurrant, melon and grapefruit juice. The subjects tended to have a preference for the one of the juices. When they were in the scanner, they were getting little squirts of juice. However, just before the squirt they were shown an arbitrary picture. What happened over time is that subjects learned to associate the juice stimuli, which were variably pleasant, with the arbitrary stimuli. What we started to see in the brain was a response that occurred to the juice stimuli which shifted back over time and over learning. Eventually, the subjects' brains were responding to the visual cues.

Chris - So in other words, you were showing them a picture, such as a circle, and then giving them a squirt of grapefruit juice. After a very short time, you showed them a picture of a circle and even without the grapefruit juice, their brains were lighting up.

John - Exactly, so in advance of the grapefruit juice happening, the brain was telling the subject that something good was going to happen to them.

Chris - You said earlier that it was useful to do this to learn how these happen, because for mankind as a whole and any kind of animal, if you can learn to associate cause and effect, then you're better at doing things in future. Now were the size of the signals that you were getting in the brain proportional to how much people liked the juice?

John - Exactly. The more they liked one of the juices, the stronger the signal was for that juice over the others. What that's telling us is that when you see a visual cue or some bit of information that's telling you you're going to get something nice, you can actually access the value that that stimulus has for you.

Chris - So if you know that writing coca Cola in a certain way tells people they're going to get something they like, you can apply that branding to some other product. This would mean that in your brain, there's going to be a transfer of the strength of how much you enjoyed that product before to the other products.

John - Yes, and that's exactly what people who want to market products try to do in advertising. They try to associate arbitrary things like the brand logo with other nice things. For example, in advertising you might show an attractive face or nice environment or scenario. By learning the association between that brand and something you find rewarding, the idea is that the next time you come along and want to make a choice, the information you've learned could bias your decision. You're more likely to choose the thing that's been associated with the thing that's nice.

Chris - What part of the brain did you see all these changes occur in?

John - We're looking at two parts of the brain. One part is called the ventral striatum, and this is deep in the centre of the brain. It's one of the parts of the brain that's important in pleasure. The other part of the brain we saw signals in was in the mid-brain. Again, this is very deep in the brain and this area contains neurons called dopamine neurons. These neurons have been implicated in reward and learning about good things. These neurons project widely around the brain, and they might be broadcasting a signal telling other parts of the brain about how nice something is. This could help the other parts of the brain to learn about that.

Chris - Now you've been able to pinpoint these parts of the brain, what implications does this have for treating human diseases in which, say, people do things too much and are addicted.

John - I think the more we understand about how these parts of the brain work, the more we can understand about how we can start to treat different diseases. These include drug addiction and other sorts of diseases where bits of the brain are not functioning properly and not telling you about how rewarding things should be, like in depression. We think this is a disorder in the ability to process rewards and unpleasant stimuli. The more we can understand how it works, the closer we are to developing treatments for these bits of the brain. This could be by using drugs that target certain parts of the brain or using certain therapies.

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